The human spine comprises individual vertebrae that interlock with each other to form a spinal column. Together, two adjacent vertebrae, their facet joints, the intervertebral disc and the connecting ligament tissue make up a motion segment unit, the smallest portion of the spine that exhibits kinematic behavior characteristic of the entire spine. The motion segment unit is capable of flexion, extension, rotation, lateral bending and translation and each component of the unit contributes to the mechanical stability of the entire unit. Trauma, degeneration, aging, disease, surgery, and the like may damage any of the components of the motion segment unit, leading to instability in the unit and causing severe pain, numbness, decreased mobility, muscle weakness and nerve damage to the patient.
One approach to treating these spinal conditions involves the use of spinal devices to stabilize and restrict, but not necessarily eliminate, the relative movement of adjacent vertebra. Spinal devices may include rigid bars, rods, plates, or combinations thereof connecting two sides of a vertebra, the adjacent vertebrae of a motion segment unit, or both. As used herein, such devices will be referred to as spinal devices or conventional spinal devices. Although these spinal devices can preserve some mobility of the motion segment units, they impart a substantial amount of rigidity to the spine. In some cases, they greatly restrict and may even eliminate certain types of motion (e.g., flexion, extension, rotation, lateral bending, axial, translation, etc.).
Another approach to treating spinal conditions involves spinal fusion. In spinal fusion, two or more adjacent vertebrae are permanently fused by forming a bony bridge between the vertebrae in order to stabilize and immobilize the motion segment unit. Ligaments, bone, disc, or combinations thereof may be removed prior to fusion. Spinal devices may be used in combination with spinal fusion to facilitate fusion of the vertebrae. As used herein, spinal devices that facilitate fusion of vertebrae are referred to as spinal fusion devices. By placing the adjacent vertebrae in their nominal position and fusing them in place, the relative movement of the vertebrae is eliminated. Thus, a fused motion segment unit is unable to undergo flexion, extension, rotation, lateral bending, axial and/or translation.
Problems are associated with either approach. The stabilization afforded by conventional spinal devices may be short-lived, since the components of the devices may fail over time. For example, any screws, pins, or hooks attaching the device to the vertebrae may eventually come loose. Another problem associated with both approaches, especially spinal fusion, involves the accelerated degeneration of vertebrae and vertebral discs neighboring the stabilized and/or fused motion segment unit. As described above, conventional spinal devices and spinal fusion devices either eliminate or greatly reduce the mobility of one or more motion segment units. As a result, vertebrae and intervertebral discs neighboring the fused or stabilized motion segment unit must accommodate an even greater degree of motion. This added stress can lead to degeneration of the neighboring vertebrae and intervertebral discs.